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Calculating Max. Demand Kilovolt-Amperes and Required Cable Current Rating

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Calculating Max. Demand Kilovolt-Amperes and Required Cable Current Rating
Calculating Max. Demand Kilovolt-Amperes and Required Cable Current Rating

A building site and electrical equipment

A building site is to have the following electrical equipment available for use:

  • Tower crane, electric motors totalling 75 kW at 415 V;
  • Sump pump, 5 kW at 240 V;
  • 60 tungsten lamps of 100 W each at 240 V;
  • 12 flood lamps of 400 W each at 240 V;
  • 20 hand tools of 400 W at 110 V.

To calculate //

  1. Find the total kilovolt-amperes to be supplied to the site if the power factor of all rotary equipment is 0.8.
  2. Find the electrical current rating for the incoming supply cable to the site.
  3. Estimate the cost of electricity consumed on the site during a 12-month contract.

Ok, let’s do it…


For rotating machinery, formulae for power in Volt Amperes [VA] goes like this:

Power [volt-amperes]

For single-phase current:

Single-phase current

For three-phase current:

Three-phase current


The results of the power calculations are given in Table 1 below. The answers required are as follows.

1. The total input power kilovolt-amperes required for site is 120.8 kVA

2. The incoming supply cable capacity at 415 volt, three-phase, 50 Hz required is:

Current

This is the input current to the site at the voltage of that cable. This is not the same as a total of the currents calculated in Table 1 as these larger numbers only appear at their reduced voltages in the relevant sub-circuits.

3. Assume that the crane, pump and tools are running for 25% of an 8-h working day, 5 days per week for 48 weeks, 20 of the tungsten lamps are for security lighting 16 h every night, and the remaining 40 tungsten lamps and the flood lamps are used for 3 h per day, 5 days per week for the winter period of 20 weeks.

The crane, pump and tools are working for:

Hours formula

The security lamps are working for:

Hours formula

The other lamps are working for:

Hours formula


Table 1 – Building site plant schedule

EquipmentPower (kW)NumberkWkVAVA
Tower Crane7517593.75415130.4
Sump Pump5156.2524026
Lamps0.1606624025
Flood Lamps0.4124.84.824020
Hand Tools0.42081011090.9
Total //98.8120.8n.a.n.a.

Table 2 – Building site energy use

EquipmentPower (kW)NumberTime (h)Energy (kWh)
Tower Crane75148036000
Sump Pump514802400
Tungsten lamps, security Lamps0.120582411648
Lamps0.1403001200
Flood Lamps0.4123001440
Hand Tools0.4204803840
Total Power Used //56528

The total energy used by the systems is found from:

kWh = number of appliances × kW per appliance × operation hours as shown in Table 2.

If electricity costs 8 p per unit (kWh) then the estimated cost for the 1-year contract will be:

Electricity cost


Reference: Building Services Engineering // Electrical Installations – David V. Chadderton (Purchase paperback from Amazon)

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About Author

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Edvard Csanyi

Edvard - Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV/MV switchgears and LV high power busbar trunking (<6300A) in power substations, commercial buildings and industry fascilities. Professional in AutoCAD programming. Present on